1. Field of the Disclosure
This invention pertains to an apparatus and process for forming a printing form from a photosensitive element and, in particular, to an apparatus and process for exposing the printing form to actinic radiation and, more particularly, to an apparatus and process for exposing a cylindrically-shaped printing form.
2. Description of Related Art
Flexographic printing plates are well known for use in relief printing on a variety of substrates such as paper, corrugated board, films, foils and laminates. Flexographic printing plates can be prepared from photosensitive elements containing a layer of a photosensitive composition such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive compositions, which may be referred to as photopolymerizable compositions, generally contain an elastomeric binder, at least one monomer, and a photoinitiator. Photosensitive elements generally have the layer of the photopolymerizable composition interposed between a support and a cover sheet or multilayer cover element. Upon imagewise exposure of the photosensitive element to actinic radiation, photopolymerization of the photosensitive composition occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. The element can be treated with a suitable solution, e.g., solvent or solvent mixture or aqueous-based solution, or treated thermally to remove areas of the photopolymerizable layer that were not exposed and leaving a printing relief which can be used for flexographic printing.
Although typically photosensitive elements for use in printing are used in sheet form, there are particular applications and advantages to using the printing element in a cylindrical form. Cylindrical printing elements have applications in flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper, and tight-fit conditions for registration, since the designs can be easily printed without print-through of the plate seam. Furthermore, such continuous printing elements are well-suited for mounting on laser exposure equipment where it can replace the drum or be mounted on the drum for digital imaging by laser exposure. Cylindrical printing elements can be continuous printing elements or seamless photopolymer sleeves that include at least a continuous layer of the photopolymerizable composition on a cylindrical support. Certain other applications use cylindrical printing elements that have one or more a planar or sheet-like photosensitive printing elements mounted onto a cylindrical support, which are often referred to as a plate-on-sleeves.
Oftentimes cylindrically-shaped photosensitive printing elements are exposed in flat-bed exposure apparatuses. Most commercial flat bed exposure apparatuses for cylindrical elements include two banks of tubular lights, each bank formed from a plurality of light tubes which form essentially a planar wall. A plurality of light tubes is necessary in order to achieve the actinic radiation energy necessary for photopolymerization of the photosensitive element to occur. The two banks of light tubes are spaced apart, opposite and parallel the other, and the cylindrical printing element which is supported at both its ends is located between the banks. In some cases, the position of one or both banks of tubes may be adjusted to create an appropriate space between the banks to accommodate exposing cylindrical printing elements with different diameters. The planar walls of light tubes can be parallel to the axis of the cylindrical printing element and typically extend beyond the diameter, i.e., width, of the cylindrical printing element. In some instances in these commercial exposure devices, each of the light tubes that form the wall has an axis that is parallel to the axis of the cylindrical printing element. In other instances, the light tubes that form the wall has an axis that is transverse to the axis of the cylindrical printing element (but still forming the planar wall). However, the distance from each of the light tubes of the bank in the first instance, or from any location on each of the light tubes of the bank in the second instance, to an exterior surface, i.e., periphery, of the cylindrical photosensitive element is different. The intensity of radiation from each of the light tubes impinging the photosensitive element is not the same due to the different distances from the each light tube to the exterior surface. Since intensity of light changes with the square of the distance between a light source and an impinging surface, even small differences in distances of the tubes to the exterior surface can significantly impact intensity and angle of the radiation impinging the photosensitive element. This may result in relatively long exposure times, exposed portions of the photosensitive element that are poorly polymerized, and/or poor quality of raised surface elements of the relief image, particularly for cylindrical photosensitive elements having relatively large diameters. These problems can be exacerbated in exposure apparatus in which the light tubes have aged and thus no longer emit at their maximum (ultraviolet) radiation.
Another problem associated with these flat-bed exposure devices is that the device typically has a large foot print, i.e., consumes a large portion of the floor area in operations. Examples of commercial flat bed exposure devices for cylindrical printing forms are available from The DuPont Company (Wilmington, Del.) by the CYREL® round Exposure Unit and the CYREL® round Light-finisher Unit. In some other commercial systems, the exposure device for the cylindrically-shaped photosensitive element is part of a multi-station automated system that not only exposes, but also treats, i.e., washes-out, and dries the element in series to produce the printing form. These exposure units have similar problems to the flat-bed exposure apparatuses described above. Additionally in these multi-station automated systems, ends of the cylindrical photosensitive element can not be covered and protected from radiation entering the ends and interior of the element and causing uncontrolled polymerization effects.
In addition there are other commercial exposure devices, so called rotary exposure devices, that include in a lid or top portion of a housing a bank of a plurality of light tubes forming a partial cylindrical wall that is parallel to the axis of the cylindrically-shaped photosensitive element. Several of these rotary exposure devices can accommodate holding cylindrical photosensitive elements within a range of diameters. But typically the bank of light tubes does not change position to maintain a constant distance from the bank to the exterior surface of the cylindrically-shaped photosensitive element, for exposing cylindrical photosensitive elements with different diameters. Oftentimes, the time of exposure is adjusted in the rotary exposure devices, as well as the flat-bed exposure devices, to accommodate cylindrically photosensitive elements of different diameters. But changing the time of exposure impacts production capability of the device, and can influence the quality of the exposed photosensitive element. If the amount of impinging actinic radiation energy is insufficient due to shortened exposure time, the photopolymerization process does not proceed as required for the photosensitive element.
In addition to the possible different diameters, cylindrically-shaped photosensitive elements have an axial length that can also differ from one element to another element. In both the flat-bed and the rotary exposure devices, the cylindrically-shaped photosensitive element can be horizontally mounted in the device by positioning each end of the axial length in a holder, which can take additional time to assure that the cylindrical element is symmetrically positioned, i.e., not skewed, and securely supported by the holders. Depending upon the size and weight of the cylindrical photosensitive element, it may require two operators to handle and mount the element into the rotary exposure apparatus.
As such, it can be difficult to establish and maintain conditions in an exposure device to provide consistent quality of actinic radiation impinging cylindrically-shaped photosensitive elements. And in particular it can be difficult to accommodate various cylindrical printing elements having different diameters and different lengths and provide consistent quality of the actinic radiation in an exposure device. The quality of the polymerization of photosensitive elements from the exposure depends on the actinic radiation energy, exposure time, and angle of the radiation. If the actinic radiation energy impinging the photosensitive element is too low, polymerization reaction may not start at all or may not occur deep enough in the layer of material which impacts the shape of small raised printing elements of the relief image. If the actinic radiation energy impinging the photosensitive element is too high such that the exposure time becomes very short, the shape of the raised printing elements is also poor. The raised printing elements have a shoulder, which is a portion of the raised printing element that transitions from a flat printing area to a sidewall, which becomes too steep, and small dots or lines do not have sufficient base and can easily chip away during printing.
Therefore, there is a need to overcome the problems of related art and to provide an improved exposure apparatus for cylindrically-shaped photosensitive elements. In particular, there is a need for an exposure apparatus that can easily accommodate various cylindrically-shaped photosensitive elements having different diameters and lengths. The exposure apparatus should also ensure safe and quick positioning of cylindrically-shaped printing elements of different lengths. Additionally, there is a need for an exposure apparatus that can provide consistent or substantially consistent intensity of the actinic radiation impinging the exterior surface of the cylindrically-shaped photosensitive element, regardless of the diameter and length of the cylindrical element.